The field of the invention pertains to the manufacture of plastic bottles and, in particular, to the processes of injection blow-molding and injection, stretch and blow-molding wherein a bottle preform is first injection-molded and, as a second step, the preform is blow-molded to the bottle shape. Between the first and second steps, the preform may be stretched to the bottom of the bottle mold as an option for enhanced bottle material properties. This method is most commonly used for the manufacture of beverage bottles from polyethylene terepthalate (PET).
U.S. Pat. No. 4,603,831 discloses a process and apparatus for forming an injection-molded preform followed by transport of the preform to a bottle mold wherein the preform is blown into the bottle shape. U.S. Pat. No. 4,233,022 illustrates further the intermediate step of stretching the preform to the bottom of the bottle mold prior to blowing the preform to the bottle shape. The preform is stretched mechanically as shown.
U.S. Pat. No. 4,007,243 discloses the manufacture of bottles from a blown preform followed by a final blowing in a second final bottle mold. And, U.S. Pat. No. 3,940,231 discloses an intermediate step of thermal treatment of the preform before the final blowing step to form the bottle.
U.S. Pat. No. 3,271,489 discloses a blow pin and neck finishing apparatus for forming the externally threaded neck of a bottle. In this patent, the bottle is formed from an extruded parison directly enclosed by the bottle mold that forms the final bottle shape. Likewise, U.S. Pat. No. 3,769,394 is directed to the neck finishing of bottles blown from an extruded parison. The excess material above the threaded top of the bottle is sheared off as a part of the neck-finishing and thread-forming plunge of the apparatus into the parison trapped in the mold.
U.S. Pat. No. 4,382,769 discloses a neck finishing blow pin and a crammer sleeve about the blow pin to pack in the parison in forming the threaded top of the bottle. The crammer sleeve also acts to sever the parison above the top and remove the severed portion when the bottle molds subsequently open. U.S. Pat. No. 4,390,338 is similarly directed to finishing the neck and threaded top of the bottle. The blow pin effects compaction of the plastic in the neck and top. A rotation collar and cutter collar remove the excess plastic from the top of the bottle as a part of the finishing process.
In the past, applicant has developed and used a modular mold system for single-step extruded parison bottle manufacture. The bottle molds comprise a split stack of partial molds. For example, the bottle volume portion, the neck and top portion, the processing dome portion, spacers as needed, and at the top of the bottle mold, a blow pin funnel are stacked. The processing dome formed above a bottle by the mold provides a means to transport the bottle to the trimmer. By means of the modular system, the overall mold height is maintained, the dome, spacer and blow pin funnel portions are standardized, and a wide variety of bottle sizes, shapes, neck and top configurations can be interchanged. A new bottle shape or top thread or other configuration can be added by merely manufacturing only the new mold parts for the volume portion or the neck and top portion of the bottle. This modular system has substantially reduced the tooling cost and therefore the per bottle cost which makes feasible relatively short-run bottle manufacture.
With a view toward expanding the advantages obtained above in manufacturing one-step extrusion blow-molded bottles to the two-step injection blow-molded process and the three-step injection, stretch and blow-molded process, the applicant has invented the improvements disclosed below. Currently, in the injection step of both processes the injection mold forms the thread or other cap fastening means and the complete sealing surface about the bottle mouth as a part of the injected preform. After the preform is separated from the injection mold, the preform is placed in the bottle mold. The bottle mold grips the preform about the thread to protect the thread from distortion and seals the preform against leakage of blowing air with a slight interference fit just below the thread. Thus, the injection mold must be critically matched with the corresponding bottle mold. The interference fit at the neck below the thread which provides the air seal can cause distortion of the plastic at the neck directly affecting the appearance and usefulness of the bottle. A new thread or cap fastening means for a bottle requires a new injection mold in addition to a new bottle mold. Also, a new height bottle requires a new injection mold in addition to the new bottle mold.
Prior art injection blow molding can also require a different preform mold and bottle mold for materials differing in shrinkage rate. The possible requirement for a new injection mold as well as bottle mold also arises from the carryover of exact dimensions from the preform to the finished bottle, particularly with respect to the cap fastening means.
The invention is directed to further modifications of the injection blow-mold and injection, stretch and blow processes for plastic bottle manufacture. The invention comprises injection molding a preform lacking a thread or other cap retention means and bottle-sealing surface at the top of the preform. The preform is substantially cylindrically shaped with a smooth circular wall extending to the mouth opening. The end opposite the mouth is closed. The invention is equally applicable to other preform cross-sections such as oblong. The preform is transferred on the injection mold core to the bottle mold wherein the bottle is blown. In blowing the bottle, the cap retention means and dome are formed and the sealing surface (sealing land) is partially formed at the location that will become the top of the bottle. The invention is equally applicable to the injection, stretch and blow process, however, typically means other than the injection mold core are used to transfer the preform.
In blowing the bottle, the neck, thread or cap fastening means, sealing land and processing dome are blown to shape with the bottle. The bottle is then removed from the bottle mold and transferred to a trimming station wherein the processing dome is removed and recycled. The trimming knife finalizes the smooth sealing surface about the bottle mouth. Thus, with the new process the preform does not require a smooth sealing surface or land at the preform mouth in the previous injection molding step. Therefore, the injection step is less critical and the entire height of the preform need not be used to form the finished bottle.
The invention is specifically directed to relatively short-run bottle production but not limited thereto. The manufacturer who specializes in such production must be able to minimize tooling and mold cost to minimize bottle production cost. With the above smooth wall shaped preform, the same relatively simple and inexpensive injection mold can be used to mold a single size preform capable for manufacture of a wide variety of bottle heights, volumes, mouth diameters and threaded or otherwise configured necks and tops.
Only the preform outside diameter is relatively fixed for the entire panoply of bottle sizes, threaded tops and shapes. With a modular blow-mold system such as that previously developed for the extrusion blow-mold process by applicant but now applied to the bottle mold of the injection blow-mold and injection, stretch and blow-molding processes, the cost of the injection mold can be significantly cut compared to a conventional injection mold with a threaded top and sealing land. In the prior art, in contrast, multiple injection molds are required for differing thread design or bottle height.
Since the bottle mold must have a thread protective top section in a conventional injection blow-mold system, two complicated top sections are provided in the tooling, one for the threaded preform injection mold and the other to protect the preform threads in the blowing step. Moreover, in the neck region just below the thread the blow-mold must be carefully fitted to prevent air leakage during blowing. This fit can cause distortion of the plastic and such distortion can cause bottle rejects.
In contrast, with applicant""s new system changing a threaded top design, size of bottle mouth or bottle height only requires the bottle mold have the new different thread configuration in the neck and top portion of the blow-mold. The preform injection mold need not be modified or removed. Moreover, the seal against air leakage during blowing is well above the sealing land of the bottle and therefore any distortion in the plastic at the air seal is subsequently removed from the bottle and recycled. Machine downtime for mold changeover is also decreased since only the bottle mold need be changed.
With applicant""s new system the preform cooling time and overall cycle time for the machine can be reduced. In a conventional process cooling the injection mold formed threads of the cap fastening means is frequently the portion of the preform making process which requires the longest cooling time and sets the overall cycle time for the bottle-making process. With the smooth-walled preform of applicant""s process, the preform cooling time is reduced, the overall cycle time is reduced and, as an added benefit, less plastic is used because the threads are blown and therefore have thinner plastic in the thread area of the finished bottle. A lighter bottle and savings in material cost are added benefits.
A further advantage results from the interchangeability of the bottle mold between injection blow-mold machines and extrusion blow-mold machines. Utilizing applicant""s modular blow-mold system for extrusion blow-molding with minor modifications to the split bottle mold, such as to the spacer and grip pieces of the modular bottle mold, the bottle mold can be used on either type of machine.
In the detailed description below, further advantages and capabilities arising from the invention are disclosed.